A 61-year-old woman who had undergone an operation for thymoma 17 years previously suddenly became dyspneic and showed bilateral pulmonary infiltrates on a chest radiograph. In the bronchoalveolar lavage fluid cells contained characteristic cytomegalic inclusion bodies, as well as cytomegalovirus DNA demonstrated by a polymerase chain reaction. Immunological findings included hypogammaglobulinemia, deficient numbers of circulating B cells, and impaired blast transformation of peripheral blood T cells in response to mitogens in vitro. Considering all of the findings, the patient was diagnosed with Good's syndrome presenting with cytomegalovirus pneumonia.